mapped for de-cat but running catted down-pipe

can this cause a problem?
i have it on because it failed the nct but its a hassle taking it on and off so i thought i would leave it on until its retest.
today i noticed when i put the foot down in third it was boosting and getting louder but just wasnt pulling at all like it should, would the catted down-pipe cause this?

mine is mapped for a decat and has the cat back in it as downpipe rotted has been there 4-5 month and no hassle but had it on the dyno and its down 23bhp :sad:have a new downpipe to go back on now so it will be grand again

think i might put the decat back on its fooking weird whats happening in third sometimes it pulls normally though.
was your old decat by any chance heat wrapped? my old one was and it rotted too, might leave it off the new one

I may be wrong
But I think I heard before ok on Irish / u.k. Cars
Not on jdm

hmm mine is jdm might try see if i can dig up more info

Putting the cat back in wont adversely affect your car at all. All the cat has done is make the exhaust more restrictive. Obviously to get the full affect of your map it is better to have the cat off. But at the moment you are not doing any harm.

thanks andy so it's def not the down-pipe but i did notice 2 things while i put the decat back on today..

where the center section and tail pipe meet the gasket was blown out because it wasn't tight enough.
the o2 sensor only goes hand tight any tighter and it loosens back up again.(even hand tight it doesn't seem like a good seal)

would any of these cause the problem in third (sometimes in third it was grand it pulled like normal but the odd time it would happen where it got louder but didnt go anywhere)

[quote author=duffmancb link=topic=37143.msg425657#msg425657 date=1361909110]
(sometimes in third it was grand it pulled like normal but the odd time it would happen where it got louder but didnt go anywhere)
[/quote]

That sounds like your clutch slipping to me.